Molecular Mechanisms of Testosterone Synthesis in Leydig Cells

Testosterone synthesis in testicular Leydig cells is orchestrated by a tightly regulated sequence of signalling events, cholesterol trafficking and enzymatic conversions. Upon luteinizing hormone (LH) binding to its G protein-coupled receptor on the Leydig cell membrane, intracellular cyclic adenosine monophosphate (cAMP) rises and activates protein kinase A (PKA). PKA phosphorylates key targets including the steroidogenic acute regulatory protein (StAR), which facilitates the rate-limiting transfer of cholesterol from the outer to the inner mitochondrial membrane. Within mitochondria, cholesterol is cleaved by cytochrome P450 side-chain cleavage enzyme (CYP11A1) to pregnenolone, which then exits to the smooth endoplasmic reticulum and undergoes sequential oxidation by 3β-hydroxysteroid dehydrogenase (3β-HSD) and cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). The final reduction to testosterone is carried out by 17β-hydroxysteroid dehydrogenase type 3. Transcriptional regulation of these steroidogenic enzymes is mediated by nuclear receptors and transcription factors such as steroidogenic factor-1, GATA-4, CREB and Nur77, whose activity is modulated by phosphorylation, co-activator recruitment and cross-talk with other pathways. Growth factors and cytokines—including transforming growth factor-β and epidermal growth factor receptor signalling—intersect with the cAMP/PKA cascade via MAPK and SMAD effectors, providing both positive and negative feedback. Intracellular Ca2+ dynamics, controlled in part by calretinin and phospholipase C-PKC pathways, further fine-tune enzyme phosphorylation, steroidogenic gene expression and mitochondrial function. Together, these interlinked mechanisms ensure that testosterone output meets physiological demands for male reproductive function, libido and systemic metabolic homeostasis.

Research from Nature Portfolio

Studies have revealed that the calcium-binding protein calretinin enhances Leydig cell steroidogenesis by augmenting intracellular Ca2+ release through phospholipase C activation and protein kinase C signalling. Elevated calretinin expression in Leydig cells increases StAR abundance, promoting greater cholesterol import into mitochondria and higher testosterone synthesis. Concomitant upregulation of phosphorylated PKC, MARCKS and CREB indicates integration of Ca2+-dependent and cAMP-dependent pathways. Loss-of-function experiments confirm that reduced calretinin impairs PLC-PKC axis activation, diminishes StAR expression and lowers testosterone output, underscoring a critical role for Ca2+ buffering proteins in the molecular machinery of steroidogenesis.

Molecular Mechanisms of Testosterone Synthesis in Leydig Cells publication trend

The graph below shows the total number of articles in molecular mechanisms of testosterone synthesis in leydig cells across all publications each year (not limited to Nature Index journals).

Technical terms

Luteinizing Hormone (LH): Pituitary glycoprotein that triggers Leydig cell steroidogenesis via a G protein-coupled receptor.

Cyclic Adenosine Monophosphate (cAMP): Second messenger that activates protein kinase A following LH stimulation.

Protein Kinase A (PKA): Serine/threonine kinase that phosphorylates StAR and transcription factors.

Steroidogenic Acute Regulatory Protein (StAR): Mitochondrial protein that mediates cholesterol transfer for steroid synthesis.

3β-Hydroxysteroid Dehydrogenase (3β-HSD): Enzyme converting pregnenolone to progesterone en route to testosterone.

G Protein-Coupled Receptor (GPCR): Membrane receptor class mediating LH signal transduction.

Mitogen-Activated Protein Kinase (MAPK/ERK): Kinase cascade integrating growth factor signals with steroidogenic pathways.

Calretinin: Calcium-binding protein that modulates intracellular Ca2+ and steroidogenesis.

Smad3: Intracellular effector of TGF-β signalling that can repress steroidogenic gene transcription.

References

1. Transcription Factors in the Regulation of Leydig Cell Gene Expression and Function. Frontiers in Endocrinology (2022).
2. Cross-talk between G Protein-coupled and Epidermal Growth Factor Receptors Regulates Gonadotropin-mediated Steroidogenesis in Leydig Cells*. Journal of Biological Chemistry (2008).
3. Transforming Growth Factor-β1 Signaling Represses Testicular Steroidogenesis through Cross-Talk with Orphan Nuclear Receptor Nur77. PLOS ONE (2014).
4. Calretinin Participates in Regulating Steroidogenesis by PLC-Ca2+-PKC Pathway in Leydig Cells. Scientific Reports (2018).